Polyethylene (PE) is synthesized by polymerizing ethylene (CH2═CH2) monomers. Because it is cheap, safe, stable to most environments and easy to be processed polyethylene polymers are useful in many applications. According to the properties polyethylene can be classified into several types, such as but not limited to LDPE (Low Density Polyethylene), LLDPE (Linear Low Density Polyethylene), and HDPE (High Density Polyethylene). Each type of polyethylene has different properties and characteristics.
Ethylene polymerizations are frequently carried out in a loop reactor using ethylene monomer, liquid diluent and catalyst, optionally one or more co-monomer(s), and hydrogen. The polymerization in a loop reactor is usually performed under slurry conditions, with the produced polymer usually in a form of solid particles which are suspended in the diluent. The slurry in the reactor is circulated continuously with a pump to maintain efficient suspension of the polymer solid particles in the liquid diluent. Polymer slurry is discharged from the loop reactor by means of settling legs, which operate on a batch principle to recover the slurry. Settling in the legs is used to increase the solids concentration of the slurry finally recovered as product slurry. The product slurry is further discharged through heated flash lines to a flash tank, where most of the diluent and unreacted monomers are flashed off and recycled.
Polymerization of ethylene involves the polymerization of ethylene monomer in the reactor in the presence of a polymerization catalyst. Suitable catalysts for the preparation of polyethylene comprise chromium-type catalysts, Ziegler-Natta catalysts and metallocene catalysts.
The use of metallocene catalysts for polymerization and copolymerization of ethylene is a relatively recent development. Processes for producing polyolefins in general and polyethylene in particular in the presence of metallocene catalysts have been described. Metallocenes are often combined with activating agents such as alumoxanes, to improve the catalytic activity of the metallocene.
A supported metallocene-alumoxane polymerization catalyst essentially comprises an inert support or carrier such as silica, on which alumoxane and metallocene are coated. In many applications for the production of polyolefins such as polyethylene, a porous support is used. The properties of such porous supports such as pore density or surface area greatly influence the physicochemical characteristics of the final polyolefin product. An increased surface area of a porous support compared to a non-porous support in theory leads to an increase in bound catalytically active sites.
Major objectives of a plant for producing polyethylene and its copolymers include the preparation of polymers having physical properties within certain specifications and the optimization of economical goals such as a specific catalyst consumption and the production rate of the plant. That is, it is desired to minimize the consumption of catalyst per ton of produced polymer, this leading to increased catalyst productivity and to a reduction in the amount of catalyst residue in the product, as well as to the maximization of the amount of polymer produced per hour.
However, although polymerization reactions in the presence of metallocene-alumoxane polymerization catalysts supported on porous supports yield polymerization products with improved physico-mechanical properties, such as uniformity, miscibility, density or molecular weight distribution, compared to polymerization reactions in the presence of non-supported catalysts, the catalyst activity of such supported catalysts is believed to be generally lower than the catalyst activity of non-supported catalysts. Hence, lower amounts of polymer per given amount of catalyst can be produced in the presence of catalysts supported on porous supports than when carrying out polymerization reactions in the presence of non-supported catalysts.
Moreover, when carrying out polymerization reactions in the presence of catalyst having low catalyst activity, higher amounts of catalyst have to be used for preparing a given amount of polymer product. In view thereof, higher amounts of catalyst may remain in the prepared polymer products, and hence such polymers may contain higher ash content. Especially, when using metallocene catalyst problems related to insufficient catalyst activity, and consequences thereof in the resulting polymer products such as to elevated ash content, may occur. For applications in food packaging or dielectric materials, high ash content is unwarranted. In these kinds of applications removal of the ash content in the polymer product needs to be carried out post-production, for instance by applying various washing and extraction techniques, which are costly and time-consuming.
In view of the above, many applications which make use of catalyst systems provided on porous supports still require improved catalyst activity to increase productivity of the polymerization reaction and hence to increase the amount of polymer product produced. Hence there remains a need in the art to provide a polymerization process for making polyolefin resin, and in particular polyethylene resin using supported catalyst having improved productivity.
It is also a need in the art to improve polymerization reactions as to reduce ash content of produced polymers which can then be used in various impurity-sensitive applications at minimal investment of additional cost and time.
In view of the above, it is an object of the present invention to provide a polymerization process for making polyolefin resin using a supported catalyst having improved activity. It is in particular an object to provide a polymerization process for making polyethylene using a supported metallocene-alumoxane catalyst having improved activity.